During assembly, and at periodic times throughout the life of a gas turbine, blade tip clearances must be measured. Proper tip clearance enables better efficacy of gas turbines and lowers harmful emissions. To date there are ways of measuring tip clearances, but the methods require the removal of the upper casing that circumscribes the turbine blade tips during measurement. Methods for measuring tip clearances in fully assembled engines are time consuming; they also rely on skill of trained measuring technicians for measurement sampling accuracy and repeatability.
There are typically two known common methods for measuring blade tip clearances. In the first method, the measurement of tip clearance is typically accomplished b y removing the upper half of the turbine or compressor outer casing, to provide access to the blades in the compressor or turbine sections of the engine. Shims are then inserted between the blades tips and the flow path side or ring segments of the lower case at the horizontal joint. Measurements are typically taken at the leading and trailing edge sides of the blade tips for every blade. These measurements are referred to as half shell measurements. This method provides very little accuracy or reproducibility because the lower case tends to change shape without the upper case in place. In addition, the measurement locations in the half shell are not necessarily representative of the local case shape at multiple angular rotational positions about the entire rotational surface swept by the turbine blade.
The second known common method for measuring blade tip clearances is by opening an inspection port in the side of the engine, through an access port, in the turbine vane carrier, through the ring segment, which is aligned with the blade path.
A contact-type probe, typically a mechanical dial indicator, is inserted into the access port of the vane carrier. Radial distance (A) from a known reference location in the carrier access port to the inner circumferential surface of the ring segment substrate facing the blade tips is known. While the blades are rotating at 6-10 rpm, a technician lowers the probe tip into the rotating path of the blade tips. Once the dial indicator starts to flutter, due to contacting the blade tips, the measurement is recorded manually, which is radial distance (B) from the same known reference location in the carrier access port to the blade tip. Clearance between one or more of the blade tips, to inner circumferential surface of the ring segment substrate, is determined by subtracting radial distance (A) from radial distance (B). Accurate reading of a fluttering dial indicator is subjective, giving only a maximum-minimum range of reading within the observed flutter range indicated by the dial indicator. Repeatability issues also arise, when readings are taken at a different time and/or by different technicians. When the fluttering, dial indicator readings, taken while the engine is in “turning gear” mode are recorded, they constitute maximum and minimum clearances for all of the blades in the blade row. It is not possible to ascertain individual tip clearances for individual blades when mechanical dial indicator readings are taken in turning gear mode. Tip clearance measurements for individual blades require manual, sequential turning of the engine rotor until each individual blade tip is oriented under the dial indicator tip. The tip gap is then measured while the blade is stationary. The gap measurement process is repeated sequentially, by moving the next desired blade under the dial indicator tip, which is time consuming. Measurement errors for each individual blade reading are possible if the dial indicator tip is not aligned on exactly the same indexed position for each blade tip. In addition, local topography variations across the surface of each individual blade tip do not assure determination of maximum or minimum clearances for each individual blade. For example, if a mechanical dial indicator reading is taken at the blade tip at the pressure (concave) side of the airfoil, its clearance may vary locally compared to the same reading taken at the suction (convex) side of the airfoil.